[C++] Support LLVM 12

[asfimport]

There is a new LLVM version available from macOS brew which causes build errors since the newest llvm version we support is 11.1

Reporter: Krisztian Szucs / @kszucs

Related issues:

• [C++][Gandiva] Add support for LLVM12 (duplicates)

_{Note: This issue was originally created as ARROW-12478. Please see the migration documentation for further details.}